To what extent is it true to say that if a Hadith is very long, it is judged to be 75% weak and 25% authentic, because the Messenger was given "Juwami' al-Kalim" (concise yet comprehensive speech)?

The authenticity or weakness of a Hadith is determined by examining its chain of narration (sanad) and applying the rules of authentication and weakening; the length of a Hadith has no bearing on its weakness. Authentic Hadiths can be long or short, as can weak and fabricated ones. The Prophet (peace be upon him) was endowed with Jawami' al-Kalim (concise speech with comprehensive meanings), which has been interpreted as the Qur'an and succinct phrases possessing numerous meanings. Not all authentic Hadiths are considered Jawami' al-Kalim.